Brakes are commonly actuated by hydraulic, pneumatic, mechanical or electrical actuating members. An electric actuating member is disclosed in U.S. Pat. No. 3,420,580, entitled, "Skid Control Device" and issued Jan. 7, 1969. Another example of an electric brake actuating member is disclosed in U.S. Pat. No. 4,653,815, entitled, "Actuating Mechanism in a Vehicle Wheel Brake and Anti-lock Brake Control System" and issued Mar. 31, 1987.
An electric motor and alternatively hydraulic motor are the primary actuating members disclosed in U.S. Pat. No. 4,602,702, entitled, "Brake Apparatus" and issued July 29, 1986. In the electric motor embodiment, a switch changes operation from a driver for the electric motor, stopping the electric motor, to a driver for a piezoceramic element positioned to act in parallel to and thereafter take over from the stopped electric motor when the load on the electric motor exceeds a predetermined value. In the hydraulic motor or piston embodiment, when the load on the piston exceeds a predetermined value, the hydraulic actuating pressure is trapped behind the piston by closing a valve, and the piston is effectively locked in place. The driver for the piezoceramic element is energized at the same time. In either instance the piezoceramic element acts in parallel to and takes over from the initially actuated motor or piston, and the initially actuated motor or piston causes no further brake applying movement of the friction braking member.
A piezoceramic member electrically actuated brake, using the piezoceramic member as the only power brake actuator, is disclosed in U.S. Pat. No. 4,623,044, entitled, "Brake Apparatus" and issued Nov. 18, 1986.
In U.S. Pat. No. 4,765,140, entitled, "Piezoelectric Servomechanism Apparatus" and issued Aug. 23, 1988, a brake has a hydraulic piston actuator that has hydraulically trapped pressurized brake fluid in the wheel cylinder chamber in an apply position. It also has a floating plate in a chamber in the hydraulic piston that, with the head of the piston, defines another hydraulic pressure chamber. Within the piston and acting axially on the floating plate are two sets of piezoelectric elements. One set reacts on another part of the piston, and the other set reacts on another piston-like working member reciprocally mounted in the piston with one side exposed to the wheel cylinder chamber. When the piezoelectric elements are electrically energized at a low voltage, they contract to a reference position. The group of piezoelectric elements acting on the floating plate and reacting on the hydraulic piston then has a higher voltage applied to it, expanding that group. The other group is then shorter than the first group, and the working member on which the shorter other group reacts moves to decrease the pressure in the wheel cylinder chamber. High voltage is then applied to the other group of piezoelectric elements and low voltage is applied to the group that previously had the high voltage applied to it. The high voltage is then applied to both groups of elements concurrently, with the displacement amount of the working member being zero, followed by concurrently applying only the low voltage to both groups of elements concurrently. This switching of voltages to both groups of elements concurrently is then followed, and is said to displace the working member by twice the displacement obtainable when only one group of elements is activated with high voltage.